In the art of optical information transmission by means of glass fiber cables, for reasons of increasing integration there is the desire for a ferrule-less fiber optical plug-in connector of high packing density. The applications of such a connector lie, for example, in the area of optical interconnects (rack-rack, backplanes), optical distributors or fiber management systems. As a result of the likely increase in the number of optical connections within these systems, the requirement for higher integration is increasing, as a consequence of which the requirements for the connecting technique are also continually increasing.
The solutions already available today are for the most part based on miniaturized variants of individual ferrule plug-in connectors or on so-called MT ferrules (MT=mechanical transfer) or compatible techniques. In today's fiber distributing systems, the distributing strips provided with the plug-in connectors make up a significant proportion of the overall space requirement.
The single-ferrule variants offer the tried-and-tested quality and reliability and can often be configured “in the field” in a simple way. However, the potential for further miniaturization and for lowering the production costs is limited. It has also already been proposed (U.S. Pat. No. 5,436,993) to connect multifiber cables to one another in a plug-in connection by means of a multiplicity of individual ferrules. However, the construction of these multiple plug-in connectors with single ferrules is complicated and elaborate and leads to comparatively high plug-in forces.
In the case of the connectors based on MT ferrules (see, for example, FIG. 7 of the initially cited U.S. Pat. No. B2-6,582,134), the high packing density is obtained, but the performance and reliability are usually only adequately ensured in multimode operation (MM). There, connectors of this type have been able to establish themselves almost as standard, in particular in the case of MM transceiver modules. Although single-mode variants (SM) are available, they impose very high requirements on the production technologies and are consequently relatively expensive to produce. Moreover, the high precision in the positioning of the fibers that can be achieved with the molded (usually injection-molded) parts of the MT ferrules is partly undermined by the cummulative tolerances over the guiding pins, which are made with relatively great dimensions in relation to the fibers. The ferrule, which is usually produced from thermoplastic and in which these pins are embedded, is subjected to high mechanical loads when it undergoes repeated plugging and unplugging and temperature fluctuations. This and the fact that individual dirt particles can endanger the functional capability of a number of fiber connections based on the large contact area, represents a significant risk to reliability.
Apart from these two approaches, examples of plug-in connectors that manage without a ferrule (ferrule-less or bare fiber or BF connectors) also already exist. Mention should be made here of the Volition™ family from the 3M company (see for example U.S. Pat. No. 5,381,498), the OptoClip® connector from the Deutsch company and, in particular, the fiber PC (FPC) connector from the NTT company (with “funnel hole” centering element; see for example the article by M. Kobayashi et al., Injection Molded Plastic Multifiber Connector Realizing Physical Contact with Fiber Elasticity, IEEE J. Selected Topics in Quantum Electronics, Vol. 5, No. 5, pages 1271-1277 (1999) and FIGS. 8A and 8B of the initially cited US-B2-6,582,134). All three of these examples are based on the principle of fibers bent in an S-shaped manner (“buckling”) and individual alignment of the individual fibers. However, only in the case of the NTT solution is a high packing density ensured from the outset. A further development of the FPC connector (EP-A2-1 241 500) uses a V-shaped profile, in which the inserted fibers are pressed against the walls by bending produced on insertion, and are thereby precisely aligned instead of funnel hole openings. A similar principle, based on “buckling”, is also realized in the case of WO-A2-02/056060. In the initially cited US-B2-6,582,134, the “buckling” is combined with an alignment of the individual pairs of fibers on insertion by means of V grooves.
All of these approaches that are based on the buckling principle assume that it is necessary to compensate for a very high linear tolerance of the fiber ends (absolutely and between the fibers), caused by the preparation and/or different thermal expansion of the connector components involved. The latter effect comes to bear in particular when standard polymers are used in the injection molding of plastics.
In this respect, the buckling principle offers a clear advantage: it is possible to compensate for linear tolerances of an order of magnitude of up to ±50-100 μm without any problem, the pressing pressure between the two buckled fiber ends remaining virtually constant.
However, the relatively strong deformation of the fibers also entails decisive disadvantages: on the one hand, the buckling does not go below a minimum bending radius, in order to avoid additional optical losses (specifically polarization-dependent losses). As a result, the minimum overall length of the plug-in connectors is limited. On the other hand, the risk of fiber rupture is increased by the flexural stresses, as a consequence of which the reliability may be impaired. Finally, the radially non-symmetric stresses occurring during the buckling of the fibers may lead to permanent changes in the optical transmission behavior. Although allowance are made for these problems by using special fibers with greater bending resistance or specially coated glass fibers, such additional measures increase the production costs and make acceptance by users more difficult.
Furthermore, in US-B1-6,371,661 it has been proposed for multifiber plug-in connectors, such as, for example, the aforementioned MT plug-in connector, in which the fibers to be connected are fixed in bores in a common ferrule, to provide an increased overhang of the fibers beyond the front sides of the ferrules of 5 to 100 μm to bring the end faces of the fibers into contact with one another with good quality, while utilizing the elasticity of the fibers. As can be concluded from comments made in U.S. Pat. No. 4,907,335, the good contact that can be achieved with this solution is obviously attributed to a “buckling” that occurs in the region of the increased overhang of the fibers.